In many processing and computing systems, magnetic data storage devices such as disk drives are used for storing data. A typical disk drive includes a spindle motor for rotating one or more data storage disks having data storage surfaces, a head arm that supports one or more transducer heads, and an actuator for moving the heads radially across the disks to enable the heads to read from and write to concentric tracks on the disks.
In general, the head is positioned very close to the corresponding disk surface. Typical clearance between the head and a smooth disk surface is about one microinch, or less. The close proximity of the head to the disk allows recording very high density servo patterns (embedded servo information) and user data on the disk.
The servo patterns are typically written into servo sectors with uniform circumferential (angular) spacing, and data sectors or blocks are interleaved between the servo sectors. The servo patterns are also arranged in radially extending servo spokes that are interspersed at regular intervals between user data areas on the disk. In addition, the servo patterns are radially close enough to allow servoing at an arbitrary radial position. At a given radius, the servo patterns include coarse identifiers and fine identifiers. The coarse identifiers provide radius and timing information when the head presents a read signal with sufficient amplitude to detect digital data. The fine identifiers are circumferentially sequential, radially staggered, single frequency bursts that provide radial position information when the head is offset from a track centerline enough to present a read signal with partial amplitude.
The servo patterns provide the disk drive with head position information to control the actuator to move the head from starting tracks to destination tracks during random access track seeking operations. Further, the servo patterns provide the disk drive with head position information to control the actuator to position and maintain the head in proper alignment with a track during track following operations when user data is read from or written to data sectors in concentric tracks on the disk surface.
In a standard manufacturing process, a head-disk assembly (HDA) of the disk drive is assembled in a clean room and then transported to a specialized servo writer where the HDA is mounted on a stabilized metrological measurement system. Then, in a time consuming process, the servo writer uses the head to write the servo patterns to the disk. The drive electronics are then assembled to the HDA and the disk drive is moved to a self-scan station where the disk drive is tested for reliable servo operation. Block errors, defects, control tracks and other information are written to the disks at this station. If the disk drive fails the self-scan tests, it is either reworked or scrapped at this late manufacturing stage.
Servo writers write the servo patterns with various processes. For example, a skip-track process writes the servo patterns at every other radius and then writes intermediate servo bursts at every skipped radius. As another example, a sync-skip process writes reference servo patterns at every other radius and then writes final servo patterns at every radius. The sync-skip process avoids the time-consuming step of measuring and compensating for head reader-to-writer offsets.
Disk drives have been developed that self-servo write the servo patterns without a servo writer. For example, an incremental two-pass self-servo write process begins with a first pass that writes reference servo patterns at a position determined by a crash-stop (the mechanical limit of the head's movement) and then servos on the reference servo patterns and writes the next set of reference servo patterns. The first pass repeats as the head moves radially across the disk, with each step servoing on the previously written reference servo patterns to write the next reference servo patterns at the next radial position. During the first pass, the servo loop has no absolute reference to ensure placement of the reference servo patterns at the appropriate radius. After the first pass finishes the complete stroke, a second pass writes the final servo patterns using the reference servo patterns to find the appropriate positions. However, the second pass substantially increases the self-servo writing time.
There is, therefore, a need for improved disk drive self-servo writing which reduces servo writer time, reduces self-servo writing time, improves performance and is simple to implement.